The present invention relates to noise attenuation systems. In particular, the present invention relates to noise attenuation systems suitable for use with gas turbine engines such as aircraft auxiliary power unit (APU) turbine engines to reduce vortex shedding.
Large commercial aircraft typically include on-board APU turbine engines, located in the tail sections of the aircraft, to provide electrical power and compressed air for systems throughout the aircraft. When an aircraft is on the ground, the primary propulsion engines of the aircraft are often shut down, and the APU turbine engine provides the main source of power for a variety of systems, such as the environmental control systems, hydraulic pumps, electrical systems, and main engine starters. The APU turbine engine may also provide power during in-flight operations, such as for electrical and pneumatic systems.
In many gas turbine engine applications, particularly those in which the engine is used in conjunction with a commercial passenger aircraft, there is a widespread demand by the airline industry to maintain noise levels below defined limits. This is particularly important at ground service stations for the aircraft, where ground crew load and unload luggage, fuel and provision the aircraft, and remove waste materials from the aircraft. Under these conditions, the aircraft APU is the turbine engine of interest.
A typical prior art APU includes a turbine shaft supported at an aft end by bearings, which are in turn supported by aerodynamically-shaped struts that provide fixed mounting to a case and/or other mounting structure relative to the airframe. These prior art APUs typically include lubricant conduits situated within the aerodynamically-shaped struts to provide lubricant supply and return paths to a bearing assembly located at a central hub that is supported by the struts. However, noise can be developed by the aerodynamic struts as exhaust flows pass by. It is desired to reduce noise generated by APU exhaust flows.